1. Field of the Invention
The present invention relates to a golf ball mold.
2. Description of the Related Art
A golf ball has 300 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight of the golf ball to accelerate the transition of a turbulent flow over a boundary layer, thereby causing a turbulent flow separation (which will be hereinafter referred to as a xe2x80x9cdimple effectxe2x80x9d). The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backwards so that a pressure resistance is reduced, resulting in an increase in the flight distance of the golf ball. Moreover, the acceleration of the transition of the turbulent flow increases a differentia between upper and lower separating points of the golf ball which is generated by a back spin. Consequently, a lift acting on the golf ball is increased.
Usually, the golf ball is formed by a mold comprising upper and lower portions having hemispherical cavities respectively. Assuming that the upper cavity is the Northern Hemisphere of a globe and the lower cavity is the Southern Hemisphere of the globe, the upper and lower portions are joined to each other over an equatorial plane (a plane including an equator). The inner peripheral surface of the mold is provided with a large number of projections. By the projections, dimples are formed on the surface of the golf ball. In other words, the shape of the dimple is obtained by inverting the shape of the projection.
A molding material (for example, a synthetic resin) leaks out of the joining portion of the upper and lower portions. Therefore, a spew is generated in a region corresponding to the equator on the surface of the golf ball. The spew is ground and removed by means of a grindstone. When the spew is generated in the dimple, it is removed with difficulty. In many cases, the dimple is not formed on the equator in order to easily remove the spew. More specifically, a projection is not provided on the joining region of the mold in many cases. Consequently, a great circle path to be a great circle which does not cross the dimple is formed on the surface of the golf ball. When the great circle path is coincident with a portion having the highest peripheral speed of a back spin (which will be hereinafter referred to as the xe2x80x9chighest speed portionxe2x80x9d), the sufficient dimple effect cannot be obtained and a flight distance is insufficient. Furthermore, in the case in which the great circle path and the highest speed portion are coincident with each other and the case in which they are not coincident with each other, the different dimple effects are obtained. Therefore, the aerodynamic symmetry of the golf ball is deteriorated. In some cases, the dimple provided in the vicinity of the great circle path is slightly deformed during the removal of a spew. If the deformation is caused, the dimple effect becomes more insufficient when the great circle path and the highest speed portion are coincident with each other.
Japanese Laid-Open Patent Publication No. Sho 62-47379 (1987/47379, U.S. Pat. No. 4,653,758), Japanese Laid-Open Patent Publication No. Hei 8-173576 (1996/173576,) and Japanese Laid-Open Patent Publication No. Hei 11-137727 (1999/137727, U.S. Pat. No. 6,123,534) have disclosed a golf ball mold in which upper and lower portions include convex and concave sections respectively and the joining surface of the upper and lower portions has a concavo-convex shape. In such a mold, a concavo-convex spew is generated. Accordingly, also in the case in which a projection for forming the dimple is provided on an equator, the spew can be generated by keeping away from the dimple. Accordingly, the spew can be removed comparatively easily. However, if the projection is positioned on the joining surface in the mold, the spew is generated in the dimple. In order to remove the spew, the projection is to be provided by keeping away from the joining surface. As a result, a large number of band-shaped smooth area (in which the dimple is not present) are generated along the joining surface in the golf ball thus obtained. The band-shaped smooth area is a comparatively large and is not coincident with the equator but is intensively provided in the vicinity of the equator, and furthermore, is extended in almost parallel with the equator. Accordingly, when the equator is coincident with the highest speed portion of the back spin, the dimple effect cannot be obtained sufficiently.
Japanese Laid-Open Patent Publication No. Sho 61-173907 (1986/173907) has disclosed a golf ball mold in which a projection for forming a dimple is provided on an equator and convex and concave sections are alternately arranged at a pitch for one projection. Since a joining surface is wavy in the golf ball mold, a band-shaped smooth area is rarely generated on the golf ball. However, it is necessary to provide a very large number of convex and concave sections in the mold. Therefore, it is hard to manufacture the mold.
Japanese Laid-Open Patent Publication No. Hei 10-127826 (1998/127826, U.S. Pat. No. 5,874,038) has disclosed a golf ball mold in which a flat surface coincident with an equatorial plane is provided between convex and concave sections. The mold is used for injection molding and a gate for injecting a material is provided on the flat surface. In the mold, the flat surface is provided between projections which are adjacent to each other over the equatorial plane. Therefore, a space between the projections cannot be reduced so that the degree of freedom of a dimple pattern design is deteriorated. Moreover, the joining surface of upper and lower portions has a double step. Therefore, it takes a great deal of time and labor to manufacture the mold.
In consideration of such actual circumstances, it is an object of the present invention to provide a golf ball mold by which a golf ball having an excellent aerodynamic characteristic can be obtained and can easily be manufactured.
In order to attain the above-mentioned object, the invention provides a golf ball mold comprising upper and lower portions having a large number of projections for dimple formation on cavities thereof,
wherein each of the upper and lower portions includes a convex section and a concave section which are alternately provided such that a joining surface to the other has a concavo-convex shape,
each of the numbers of the convex sections of the upper portion, the concave sections of the upper portion, the convex sections of the lower portion and the concave sections of the lower portion is three to five,
each of the convex sections has two to eight projections on a first line from the joining surface, and
at least one of the projections on the first line from the joining surface in each of the convex sections is constituted such that an equation (I) is established:
Rxc3x970.8xe2x89xa6Lxe2x89xa6Rxc3x971.2xe2x80x83xe2x80x83(I)
(In the equation (I), R represents a radius of the projection and L represents a linear distance in a vertical direction between a center of the projection and the joining surface).
In a golf ball obtained by the mold, a large band-shaped smooth area is not present. In addition, also in the case in which the edge of a dimple is deformed by removing a spew, a deformed portion is not arranged along a great circle. Accordingly, the golf ball is excellent in an aerodynamic characteristic. Moreover, the mold can be manufactured comparatively easily, and furthermore, a dimple pattern design is less restricted.
It is preferable that the projection should be provided such that a surface of a golf ball to be molded has no great circle path to be a great circle which does not cross a dimple. A golf ball obtained by the mold has a more excellent aerodynamic characteristic.
It is preferable that a gate for injecting a molding material should be provided in a position where the joining surface crosses an equatorial plane. Although the mold is used for injection molding, it has a high degree of freedom of a dimple pattern design and can be manufactured easily.
It is preferable that a joining surface to the other in each of the upper and lower portions should have a convex surface present in the convex section in almost parallel with an equatorial plane, a concave surface present in the concave section in almost parallel with the equatorial plane, and a boundary surface positioned between the convex surface and the concave surface. All the convex surfaces have areas equal to each other, all the concave surfaces have areas equal to each other, and the convex and concave surfaces have areas equal to each other. In the mold, pressurizing force generated when molding a golf ball is uniformly dispersed over the mold. Therefore, the golf ball thus obtained has a uniform quality.
It is preferable that a difference in a height between the highest convex section and the lowest concave section should be 2.0 mm or less in each of the upper and lower portions. The mold can easily be manufactured and a golf ball can easily be taken out of the mold after molding.